A computer implementing a virtual memory system typically employs a certain amount of "physical" memory composed of relatively fast semiconductor RAM devices, along with a much larger amount of "virtual" memory composed of hard disk, where the access time of the hard disk is perhaps several hundred times that of the RAM devices. The virtual memory management scheme uses an operating system such as UNIX.TM. along with hardware including a translation buffer, and is well known. In multi-tasking operating systems where more than one program runs at the same time, each running in a time slice of its own, each program appears to have an entire memory space to itself. To make room in the physical memory to run a new program, or to allocate more memory for an already-running program, the memory management mechanism either "swaps" out an entire program (process) to disk memory or "pages" out a portion (page) of an existing process to disk.
Transferring data to and from disk memory is very slow compared to the transfer time to main memory, and so "solid state disks" (composed of semiconductor RAMs like the main memory) have been used as a substitute for magnetic disk to improve system performance. This is at a much higher cost per megabyte of storage, however, due to the cost of semiconductor RAMs. Data compression has not been used because of the variable-length record problem as discussed below, i.e., compressed data blocks are of variable size, making random access of compressed "pages" of data impractical.
In memory-constrained environments a certain percentage of the pages of the total virtual storage in use will not actually be resident in the machine's physical memory frames. These pages, when referenced, must be brought into memory from the paging space (which is kept on physical disk) in a page fault operation. Such page faults are expensive because physical disk operations typically take tens of milliseconds, all of which time the faulting application must wait.
Ideally, a configuration will have sufficient real memory to avoid continuous page fault activity. Because of both the expense of memory, and the difficulty of sizing memory requirements, this is not always the case. Packaging requirements for portable machines (e.g., laptops) are likely to exacerbate this problem.